Dave (PG-13) may be the first presidential comedy that’s rooted not merely in a specific administration but in its first 100 days: It’s a blissed-out satirical fantasy about the dawn of Clintonism. Directed by Ivan Reitman, who after years of whiz-bang entertainments like Ghostbusters and Kindergarten Cop now reveals an unexpectedly gentle side, this agreeable flight of fancy features Kevin Kline in a dual role. He’s Bill Mitchell, cynical, ill-tempered President of the United States, and Dave Kovic, a nice, gawky, unassuming chap who runs a temp agency in Baltimore, and happens to be a dead ringer for the President. Because of this eye-popping coincidence, he is tapped, as a cautionary decoy, to fill in for Mitchell while he exits a public function. Dave does the job and is being chauffeured back to obscurity when the President suffers a stroke (while making love to his mistress). That’s when the evil White House chief of staff (Frank Langella) gets an inspiration: He’ll recruit Dave to fill in for the President full-time, turning him into an Oval Office puppet.

Naturally, it doesn’t work out that way. The moment Dave enters the White House, he begins to infuse it with his exuberant, ordinary-guy personality. He indulges his taste for hero sandwiches and plays with his dog on the lawn. He calls in an accountant pal (Charles Grodin) to look at the budget (”Who does these books?” says Grodin with peerless exasperation). He thaws the frozen heart of the First Lady (Sigourney Weaver, in a Nancy Reagan ‘do), and for a few minutes Dave turns into a Beltway Sommersby.

Mostly, Dave has fun, and the movie mirrors his mood; you can feel the kick Reitman must have gotten from setting this mellow farce amid the corridors of power. The White House seems cushy and inviting here, like a grand hotel — it’s easy to imagine flopping around on the presidential bed — and the warm way the setting is visualized melds with the film’s impish attempt to show us the human side of America’s ruling elite. As the President, Dave is joyful, direct, real. He actually says what he believes. Is it any wonder the country responds to him?

In this movie, the rebirth of hope in American politics becomes both a joke and a giddy fairy tale. Kline plays President Mitchell as a hard-shelled aristocratic phony glued to his TelePrompTer. In other words, he’s a broad projection of everything Americans rejected in George Bush. And Dave? He’s the anti-Bush, the President as common man-industrious, compassionate, junk-food- loving. Like Tom Hanks in Big (cowritten by Gary Ross, who wrote this movie solo), he’s a sweet-souled imposter who, because of his guileless nature, turns out to be far better at his job than the calculating adult he’s impersonating. Dave incarnates the emotional quality that helped get Clinton elected-the sense that, after decades of corruption (Nixon), incompetence (Carter), and heartless media gamesmanship (Reagan/Bush), he was a leader who could reflect the better side of our natures.

As nuts-and-bolts politics, Dave is softheaded hokum. When Dave starts to formulate ”legislation,” what his plan boils down to is: Let’s get some more jobs for people! And let’s fix up the homeless problem, too! The naiveté is almost too much to fathom. (And when will people in Hollywood stop using ”helping the homeless” as an instant metaphor for caring?) Nevertheless, it would be silly to take Dave on such a literal level. The movie isn’t concerned with policy. It is, rather, a populist farce that wins its laughs — and, in Dave’s big speech to Congress, its lump in the throat — by revealing just how far we’ve strayed from having any faith in our leaders.

The movie is studded with cameos by media pundits (the McLaughlin Group, etc.) and U.S. Senators, and I only wish these bits were as witty in execution as they are in theory; their main purpose seems to be to let us know the film is hip enough to make inside references. Langella, though, has a triumph as the glum conniver Bob Alexander, who’s like a synthesis of all the overbearing powermongers who’ve skulked through the White House over the last 20 years. Even his casual comments have a silky menace. And Kline has grown into a masterful sly comedian. As Dave, the sort of guy who sings ”Oklahoma!” while peddling his bike from work, Kline never tips his hand, never lets himself seem too boobish or innocent. He does a deadpan star turn; he cuts through the pomposity of high office as if it were second nature. For all its modest charm, Dave is a true throwback to the Capra days, a political comedy just cockeyed enough to triumph over cynicism. B+